The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics that are used to couple digital signals from a source to an isolated receiver magnetically and, more particularly, to circuits used to monitor performance of such coupling structures.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as field sensors. Magnetometers and other magnetic field sensing devices are used extensively in many kinds of systems including magnetic disc memories and magnetic tape storage systems of various kinds. Such devices provide output signals representing the magnetic field sensed thereby in a variety of situations.
One use for such magnetic field sensors is the sensing of magnetic fields generated by electrical currents in a conductor as a basis for inferring the nature of such currents giving rise to these fields. While this has long been done for magnetic fields generated by substantial currents, such sensing becomes more difficult to accomplish in smaller ranges of currents that include relatively small currents. The need for sensing fields due to such currents arises, for instance, in situations where the currents generating the field to be measured are provided merely as a basis for conveying signal information rather than for transmitting substantial electrical energy.
Such a situation occurs in many medical systems, instrumentation systems and control systems where there is often a need to communicate signals to system portions over signal interconnections from an external source or from another portion of the system. Often, the conductors carrying signal currents for such purposes must be electrically isolated from the portion of the system containing the sensor arrangement for those signals to measure the resulting magnetic field. As an example, a long current loop carrying signal information in the loop current may, through lightning or static electricity discharges, become subject to having large voltage potentials relative to ground developed thereon. Such potentials must in many instances be kept from the signal sensing and receiving circuitry to avoid damage thereto even though that circuitry must still be able to capture the signal information contained in the loop current.
Signal isolators for these purposes are often preferably formed in monolithic integrated circuit chips for reasons of cost, convenience and system performance. In such an arrangement, one or more solid state magnetic field sensors are used to detect the magnetic fields provided by the currents containing the signals. One effective arrangement that has emerged in these circumstances for signal isolation in both hybrid integrated circuits and monolithic integrated circuits comprises an input conductor, typically in some coiled electrical current conductor configuration, and a current sensor both supported on a substrate adjacent to and spaced apart from the input conductor so that they are electrically isolated from one another but with the current sensor positioned in those magnetic fields arising from any input currents appearing in the input conductor. The sensor is often connected to an amplifier to form a current determiner. Such an isolator or determiner is an attractive device for these purposes in being both rapid in operation and economic low in cost, and has been disclosed in U.S. Pat. No. 5,831,426 to W. C. Black and T. M. Herrmann entitled xe2x80x9cMagnetic Current Sensorxe2x80x9d, and in U.S. Pat. No. 6,300617 to J. M. Daughton, R. T. Fayfield, T. M. Hermann and J. F. Stokes entitled xe2x80x9cMagnetic Digital Signal Coupler,xe2x80x9d both of which have been assigned to the same assignee as the present application and both of which are hereby incorporated herein by reference.
These current sensors are typically magnetoresistive effect based sensors. They are typically formed with an intermediate thin-film layer of a nonmagnetic separating material having two major surfaces on each of which an anisotropic ferromagnetic thin-film is positioned which has been found to lead to a xe2x80x9cgiant magnetoresistive effectxe2x80x9d in the sensor if the thickness of the ferromagnetic thin-films in the intermediate layer in such a xe2x80x9csandwichxe2x80x9d structure have been made sufficiently small, or to a magnetically controlled tunnel diode.
In such monolithic integrated circuit based signal isolators, power dissipation can be reduced along with the risks of electromigration of the conductors therein, and so the reliability thereof can be increased, by using a differentiating input driver circuit, or at least an input driver circuit that approximates differentiation, for operating the input coil rather than allowing the original current signal carrying the information to be transferred to circulate therethrough. Such a differentiating driver circuit, or differentiating-like driving circuit, generates either a set or reset pulse in the input coil whenever the digital input signal transitions between low and high logic state current levels, or vice versa, and so in the magnetic field generated thereby. The inherent memory characteristics of certain xe2x80x9cgiant magnetoresistive effectxe2x80x9d based sensors, or the use of latching electronics in the receiver circuits connected to other kinds of xe2x80x9cgiant magnetoresistive effectxe2x80x9d sensors (or even to such sensors with inherent memory characteristics), maintains the output of the signal isolation in its most recent logic state until the receiver circuit detects a change of logic state in the input signal, i.e. another set or reset pulse. High common mode rejection capabilities are typically designed into such signal isolators to ensure that the output responds only to such logic state changes in the input circuitry during normal operation.
However, under abnormal power supply performance conditions, or during circuit operation initiation after first switching on the digital isolator for a new use session, there exists the possibility of the signal isolator output to exhibit an incorrect logic state. An incorrect logic state can result in erroneous data, damaged equipment, or can be a safety hazard when the device is used in man-machine interface equipment. Also, since the signal isolator operates on the rising and falling edges of the input signal transistions between logic states, i.e. is an edge triggered device, it is susceptible to spurious edge triggers leading to the risk of the output data being erroneously shifted in logic states so that it no longer tracks the logic states of the input signal. Spurious edge triggerings of the isolator can be caused by ground transients, electromagnetic interference or unexpected transients through the power supply and the like. This risk is especially high in low data rate input signals or low rate of change between logic states in the input signal where the time between desired logic state transitions, or in completing a transition, becomes relatively long thereby increasing the time for such unwanted occurrences.
Such problems have been sought to be overcome by transmitting an updating signal, with respect to the current input logic state, to the receiver circuit across the isolation barrier between the input coil and the receiver sensor at a predefined rate. Alternatively, both input signals and timing (xe2x80x9cclockingxe2x80x9d) signals have been transmitted across the isolation barrier as a basis for determining occurrences of desired logic state changes. Such arrangements, however, lead to increased power dissipation and increase complexity of the circuitry both on the input coil side of the isolation barrier and on the receiver side of that barrier. Thus, there is a desire to provide an arrangement that monitors the logic state situation on the input coil side of the isolation barrier and to provide some assurance as to the correct logic state on the receiver side of the isolation barrier with respect to the current or last logic state of the input signal on the input coil side of that barrier.
The present invention provides a transformable signal transmission system for providing electrical energization to a load electrically connected to a system output based on input signals provided to a system input except during occurrences of a selected condition which occurrences initiate a transmission transformation during which an alternative energization is provided to the load. The system has a condition determiner for determining the occurrence or absence of the selected condition and provides a first condition signal upon absences of the selected condition and a second condition signal upon occurrences of said selected condition. A pulser provides the system output and the system input and is also connected to an oscillator. The condition determiner control output is coupled to at least a selected one of the oscillator and the pulser. This pulser is capable of providing electrical energization pulses at the system output based on the system input signals when the first condition signal is provided on the condition determiner control output, and of providing alternative electrical energization at the system output based on signals supplied by the oscillator when the second condition signal is provided on said condition determiner control output.
The system output can be connected to a coil to provide pulsed magnetic fields about a magnetic field sensor positioned on the other side of an electrical insulating layer from that coil. The selected conditions include magnitudes of operating voltage supplied to the signal transmission system being less than a selected monitoring value and durations of times between occurrences of pulses at the pulser output exceeding a corresponding selected monitoring value.